The prior art has established numerous techniques for distance measurement. For example, U.S. Pat. No. 3,757,285 discloses an acoustic range finder. Compensation is provided for factors affecting speed of travel of an acoustic pulse through a medium, such as temperature, pressure, humidity, etc. of the medium. Compensation is specifically provided by changing the frequency of a reference oscillator used in the system.
Such techniques may be used for height measurement for a descending object, such as a parachutist or a payload, as noted in U.S. Pat. No. 4,618,110. Therein, a distance from the surface measured by ultrasonic devices is compared with predetermined values to provide a quick release lock for automatically releasing the parachute from the load, or person, when the distance is zero.
A system for measurement of both distance and velocity of approaching targets at short distances, usable for approaching aircraft and in an airplane landing service, is disclosed in Marukawa et al., "An Accurate System of FM-CW Radar for Approach Using Phase Detection", Electronics and Communications in Japan, Vol. 58-B, No. 2, pp. 65-73, February 1975. Therein, a single Doppler system is used for both distance and velocity measurement, requiring performance of complex calculations in order to provide the results from measurement of the frequency of beat signals and from signal phase changes. The publication appears to be directed at solving a problem of a step error specific to distance measurement using FM radar rather than at correcting the result of the measurement for intrinsic errors due to movement of the object. The system thus relies on radar transducers for measuring short distances above the ground, which are unreliable and are extremely expensive.
Moreover since preferred, less expensive, height measurement systems are acoustic in nature, utilization of the disclosed Marukawa et al. system requires complete replacement of an existing system by a more expensive system rather than improving the accuracy of the existing system by adding a less expensive and more simply implemented modification. Thus, rather than correcting deficiencies in existing sonic systems, the Marukawa et al. disclosure is directed at attaining similar improvement in accuracy by using systems wherein the moving-distance signal is measured to an accuracy approximately equal to a half wavelength at the carrier frequency of the Doppler radar based distance measuring signal.
Japanese publication 2-62991 (A) measures moving distance and speed of an object by providing a doppler radar main body 10 with a high frequency or ultrasonic wave oscillator 11. From the brief English language disclosure therein, the nature of the interaction between a Doppler signal output from an amplifier 15 and the ultrasonic oscillator is not apparent. However, it appears from the disclosure that the Doppler frequency is used to determine the distance by which the object moves, as noted at lines 6-8 of the "Constitution" section thereof. Moreover, from the last two lines of the "Constitution" section it seems that speed is calculated, rather than being detected and used to compensate for errors in the distance computation.
U.S Pat. No. 3,838,421 provides an interaction between radar and sonic devices. However, a radar range pulse is enabled by a sonic pickup pulse from a vibration transducer detecting wheel contact with the airport surface in order to determine a range difference between the actual aircraft landing touchdown and the theoretical touchdown point on the runway.
A speed forecasting circuit is disclosed in Japanese publication 1-182776 (A). Therein, position and speed information of a tracked moving object are stored and converted into coordinates for the object. The system outputs forecast speed information of the moving object as a projection from previous forecast information.
There is accordingly a need in the prior art to provide an improved accuracy dynamic distance short range acoustic measuring system.
Still a more particular need exists in the prior art for correcting errors occurring in a height measurement because of vertical movement during a relatively long period of time used for height measurement by an ultrasonic device.
There is yet a further need in the prior art to be able to correct for such long measurement periods by using short measurement periods available for velocity measurement devices.
There is a more specific need in the prior art to provide a hybrid system which corrects height measurements made by an ultrasonic height measurement system for vertical velocity by using a Doppler radar system operating in shorter measurement periods and relatively unaffected by the distance measurement.